Hybrid vehicles that have an electric motor-generator are often configured to implement regenerative braking to slow the vehicle by operating the motor-generator as a generator. The motor-generator converts rotational torque of the vehicle axle into electrical energy stored in an energy storage device, such as a battery. The availability of regenerative braking is variable as it depends upon the charge power limit of the energy storage device. For example, if the charge power limit of the energy storage device is less than the required braking power, then regenerative braking will not be able to provide all of the desired braking power, and the friction brakes of the vehicle will be required to absorb at least some energy. If, during a braking event, the energy storage device reaches a predetermined maximum state of charge, the regenerative braking is ceased and the system must increase the level of friction braking to meet the braking demand. Although suitable for meeting the level of braking demand, these systems can have an uneven feel when such a transition from regenerative to full friction braking occurs. Additionally, because the availability of the second form of braking (i.e., the regenerative braking) is variable, the friction braking system must be able to bear a substantial amount of load, adding cost to the system. For example, an additional hydraulic pump is often required on such braking systems to increase the braking capacity.
Some vehicles have a dedicated exhaust valve in the exhaust system that can be moved to restrict exhaust flow, adding torque on the engine to slow the engine, providing an additional source of braking power. The dedicated exhaust valve is used only for this purpose on such vehicles, adding expense to the vehicle.